1. Field of the Invention
The present invention relates generally to a method for imaging of biological tissues and in particular to a method for imaging tumors using a magnetic resonance imaging device to monitor an embolization procedure.
2. Description of the Related Art
Tumors may be surgically removed, or resected, if possible. However, some tumors are unresectable and so other procedures must be used. One such procedure is embolization, or the selective occlusion of blood vessels to slow or stop the blood flow to the tumor so as to reduce tumor size. One area for use of embolization is for the treatment of liver lesions, an example of which is hepatocellular carcinoma, one of the more common liver tumors. The procedure for embolization that uses a catheter to deliver the embolic agent to an artery is termed transcatheter arterial embolization. Transcatheter arterial chemoembolization (TACE) is an interventional procedure in which chemotherapeutic medications and embolic particles are injected via a catheter into a blood vessel supplying the tumor.
The transcatheter arterial embolization (TAE) and transcatheter arterial chemoembolization (TACE) procedures preferentially deliver embolic agents to a hepatocellular carcinoma (HCC) via catheters positioned within the hepatic arteries. Currently, data supporting optimum embolic endpoints for the TAE and TACE procedures (sub-stasis endpoints or alternatively complete stasis of antegrade blood flow) remains conspicuously lacking. Complete stasis of flow may increase normal liver toxicity and potentially induces release of growth factors promoting tumor growth. Embolization to levels beyond therapeutic benefit may accelerate liver decompensation and subsequent failure. Determination of end points for the embolization process is needed. However, accurate intra-procedural tumor perfusion measurements are currently not possible with conventional digital subtraction angiography (DSA) imaging techniques, thus representing a significant limitation for evaluating the efficacy of sub-stasis endpoints.
The selection of optimal embolic endpoints during TAE and TACE remains controversial. Rational arguments can be made in support of either of stasis or sub-stasis endpoints but there currently exist no quantitative data to support such hypotheses. One reason for the lack of such data has been the inability to reproducibly assess sub-stasis endpoints of tumor perfusion using conventional DSA. Monitoring of TAE with DSA relies upon highly subjective assessment of reductions to antegrade blood flow. Although the spatial resolution of DSA permits superior visualization of vascular anatomy, small or hypo-vascular tumors can be poorly visualized. While the recently introduced 3D x-ray angiography system has improved turn or visualization for optimal catheter placement, quantitative tumor perfusion measurements have yet to be demonstrated.
In recent studies, hepatic perfusion measurements were performed using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) with intravenous (IV) injection of gadolinium-based contrast agents. These perfusion measurements demonstrated good correlation to radio-labeled microsphere measurements in a rabbit model (r=0.93) and to thermal diffusion probe measurements in a pig model after portal vein occlusion (r=0.90). Additional studies recently used DCE-MRI to evaluate alteration to HCC perfusion after a TACE procedure and fibroid perfusion after a uterine fibroid embolization procedure. However, in these studies, pre- and post-therapy DCE-MRI was performed separately from the interventional procedures.
To use DCE-MRI for intra-procedural monitoring of TAE and TACE procedures, perfusion measurements must be performed iteratively at each stage of embolization. Recent developments of the magnetic resonance interventional radiology (MR-IR) system (a digital subtraction angiography (DSA) suite adjacent to a dedicated MRI scanner) should facilitate acquisition of functional imaging data for infra-procedural guidance. However, with IV contrast injection, the number of DCE-MRI measurements is limited by cumulative dose and the requisite wash-out times between injections. Transcatheter intra-arterial (IA) injections permit reductions in contrast dose while increasing liver tumor conspicuity and reducing wash-out times. For DCE studies, targeted delivery of the contrast agent to the hepatic artery should simplify the evaluation of enhancement curves by requiring consideration of only a single arterial input supply (as opposed to complex dual-supply liver perfusion models). TRanscatheter Intra-arterial Perfusion (TRIP)-MRI may be ideal for the iterative detection of perfusion reductions resulting from super-selective injection of embolic materials during TAE and TACE procedures.